They have faith in a Christ of their own making, which is the same as saying, they have faith in no Christ at all, but just in the figments of their own minds.
Every sin consists formally in aversion from God, as stated above. Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him. Nor is it possible for one who has a false opinion of God, to know Him in any way at all, because the object of his opinion is not God. Therefore it is clear that the sin of unbelief is greater than any sin that occurs in the perversion of morals. (St. Thomas Aquinas, Summa Theologica, II-II, Q. 10, Art. 3)